FIG. 12 illustrates a conventional electric power steering apparatus disclosed in, for example, Japanese Patent Application Laid-Open No. 4-31171. A motor 1 that generates an auxiliary steering force is controllably driven by a motor drive circuit 2. The motor drive circuit 2 is in the form of an H-bridge circuit using power transistors. The current flowing through the motor 1 is detected by a motor current detecting circuit 3. The voltage across the terminals of the motor 1 is detected by a motor terminal voltage detecting circuit 4.
A clutch 5, which mechanically disconnects the motor 1 from a steering shaft, is driven by a clutch drive circuit 6.
A torque sensor 7 detects the steering force applied by a driver and a vehicle speed sensor 8 detects the cruising speed of the vehicle.
A microcomputer 9 reads the outputs of the torque sensor 7, vehicle speed sensor 8, and others. The microcomputer 9 controls the motor 1 such that the motor 1 generates an optimum auxiliary steering force in accordance with the driving conditions, and detects the line faults on the basis of the outputs of the motor terminal voltage detecting circuit 4 and others. The power supply voltage from a battery 10 is supplied to the motor 1 via the motor drive circuit 2 and to the microcomputer 9 via an ignition switch 11.
The operation of the conventional apparatus will be described. When the driver throws the ignition switch 11 to the ON position, the microcomputer 9 receives electric power and reads the outputs of the torque sensor 7 and vehicle speed sensor 8. In accordance with the cruising conditions of the vehicle and the steering operation performed by the driver, the microcomputer 9 computes an optimum auxiliary steering force that the motor 1 should generate.
The motor 1 is a DC motor where the output torque is proportional to the motor current. Therefore, the motor current indicates the generated auxiliary steering force. Thus, the computed steering force is equivalent to a target current that should be supplied to the motor 1.
The microcomputer 9 reads the motor current detected by the motor current detecting circuit 3, and performs feedback-controlling the motor 1 such that the detected motor current becomes equal to the target current of the motor 1, thereby computing a voltage that should be applied to the motor 1. Then, the microcomputer 9 sends the computed voltage to the motor drive circuit 2 to drive the motor 1.
Meanwhile, the motor terminal voltage detecting circuit 4 detects the voltage across the terminals of the motor 1. The microcomputer 9 compares the computed voltage (target voltage) with the detected voltage. If the difference between the target voltage and the detected voltage remains larger than a predetermined value for a time period longer than a predetermined value, then it is determined that a line fault has occurred.
For example, when the detected voltage is lower than the target voltage, if the difference between the two voltages exceeds a predetermined value and lasts longer than a predetermined time length, it is determined that, for example, a ground fault has occurred on the power line(s) connected to the motor 1.
However, the motor 1 generates a voltage (back electromotive force) proportional to its rotational speed. When the motor 1 is energized to rotate, the back electromotive force causes a difference between the target voltage and the detected voltage even though the steering apparatus is operating normally, there being a possibility of the microcomputer 9 performing an erroneous detection of line faults.
An electric power steering apparatus may encounter a case where the motor 1 is not supplied with electric power but is driven in rotation by its load, i.e., the rotational force of tires. For example, when the vehicle is running with the steering wheel rotated through a given angle, if the driver moves his hands off the steering wheel, the tires return to their neutral positions (self-aligning torque), causing the steering wheel to return to its neutral position so that the motor 1 rotates to generate a back electromotive force independently of the target voltage.
Such a case may occur frequently, and even if the target voltage is zero volts, a voltage due to the back electromotive force appears across the terminals and is detected. Therefore, conventional methods of detecting the line faults of an electric power steering apparatus are highly apt to detect erroneous faults. Thus, a false detection of line faults will occur frequently if a line fault in the electric power steering apparatus is to be detected in terms of the difference between the target voltage and the detected voltage. Such a frequent false detection is detrimental.
When taking some measure to prevent such a false detection, it is necessary to allow, for example, a very long time from the detection till it is determined that a line fault has actually occurred and a fail-safe operation is carried out subsequently. However, this is detrimental since such a long time impairs the ability to detect line faults.
As mentioned above, with a conventional electric power steering apparatus, the conventional fault-detecting methods including the aforementioned method suffer from a drawback that the back electromotive force of the motor 1 causes frequent false detections of line faults if a fault of the motor 1 is to be detected in terms of the terminal voltages.
The present invention was made to solve the aforementioned drawbacks. An object of the invention is to provide an electric power steering apparatus where faults such as a ground fault of the power lines connected to the motor can be detected without false detections due to, for example, the back electromotive force of the motor.